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Chemical Characterization and Spatial Distribution of Microplastics in the Surface Water of the Arun River Basin, Central Himalaya, Nepal
Summary
Researchers detected microplastics at all 14 sampling sites across the Arun River in Nepal's central Himalayas, finding average concentrations of 30 particles per liter with elevated levels near cropland and construction zones, and identifying polyethylene and PET as the dominant polymer types.
Glacier-fed Himalayan Rivers are vital lifelines for South Asia, sustaining ecosystems, agriculture, and millions of people across national boundaries. Although microplastic pollution is reported in several water systems, the transboundary rivers in the central Himalayas remain critically understudied. This research investigates the chemical characteristics and spatial distribution of microplastics in the surface water of the Arun River, a major transboundary tributary of the Ganges system, spanning China, Nepal, and India. Surface water samples (n = 28) were collected in duplicate from 14 sites across the Arun River Basin, Koshi Province, Nepal, selected based on land use and land cover, spatial distribution, and anthropogenic activities. Microplastics were detected in all samples, with an average concentration of 30 ± 3.2 particles per liter (range: 23–35 particles/L). Microplastic concentrations were notably elevated in proximity to cropland and built-up zones, with peak levels recorded at sites influenced by construction activities and agricultural runoff. Intriguingly, comparable concentrations were also detected at relatively undisturbed locations, indicating the potential for long-distance transport mechanisms. Most particles were black (78%), fragment-shaped (69%), and small-sized (32–100 μm; 95%). Dominant polymers included polyethylene, polyethylene terephthalate, polypropylene, and polystyrene. Multivariate analysis and land use mapping indicated higher microplastic loads in urbanized and construction-impacted zones, while tributary inflows contributed to dilution. Unlike global patterns of downstream accumulation, microplastic concentrations in the Arun River peaked in its middle reaches, reflecting localized land use dynamics. These findings highlight the influence of human activities and call for targeted interventions to reduce plastic pollution in transboundary Himalayan River Systems.